1. Field of the Invention
The present invention relates to a zoom lens system, an imaging device and a camera. In particular, the present invention relates to: a high-performance zoom lens system that has a remarkably reduced overall length of lens system (the optical axial distance measured from the object side surface of the most object side lens element to the image surface) and hence a reduced size and still has good balance between a wide view angle at a wide-angle limit and a high zooming ratio; an imaging device employing this zoom lens system; and a thin and compact camera employing this imaging device.
2. Description of the Background Art
Remarkably strong requirements of size reduction and performance improvement are present in digital still cameras and digital video cameras (simply referred to as digital cameras, hereinafter) provided with an image sensor for performing photoelectric conversion. In particular, from a convenience point of view, digital cameras are strongly requested that employ a zoom lens system having a high zooming ratio and still covering a wide focal-length range from a wide angle condition to a highly telephoto condition. On the other hand, in recent years, zoom lens systems are also desired that have a wide angle range where the photographing field is large.
As zoom lens systems having a high zooming ratio and zoom lens systems having a wide angle range as described above, in the conventional art, various kinds of zoom lens systems have been proposed that has a four-unit construction of positive, negative, positive and positive in which in order from the object side to the image side, provided are a first lens unit having positive optical power, a second lens unit having negative optical power, a third lens unit having positive optical power, and a fourth lens unit having positive optical power.
Japanese Laid-Open Patent Publication No. 2005-326743 discloses a zoom lens having a four-unit construction of positive, negative, positive and positive and having an aperture diaphragm located between the second lens unit and the third lens unit, in which in magnification change from a wide-angle limit to a telephoto limit, the interval between the first lens unit and the second lens unit increases while the interval between the second lens unit and the third lens unit decreases, in which the ratio between the focal length of the entire system and the maximum image height is set forth at a wide-angle limit, and in which the second lens unit is composed of three lenses, in order from the object side to the image side, consisting of a negative lens with the larger curvature surface facing the image side, a positive lens with the larger curvature convex surface facing the image side, and a negative lens with the larger curvature concave surface facing the object side.
Japanese Laid-Open Patent Publication No. 2006-078979 discloses a zoom lens having a four-unit construction of positive, negative, positive and positive, in which in magnification change from a wide-angle limit to a telephoto limit, the interval between the first lens unit and the second lens unit increases while the interval between the second lens unit and the third lens unit decreases, in which the ratio between the focal length of the entire system and the maximum image height is set forth at a wide-angle limit, and in which the second lens unit is composed of three lenses, in order from the object side, consisting of a negative lens with the larger curvature surface facing the image side, a positive lens with the larger curvature surface facing the image side, and a negative lens with the larger curvature surface facing the object side, with an air space between the individual lenses.
Japanese Laid-Open Patent Publication No. 2006-235062 discloses a zoom lens having a four-unit construction of positive, negative, positive and positive, in which in magnification change from a wide-angle limit to a telephoto limit, the first lens unit and the third lens unit move to the object side while the aperture diaphragm moves independently of the adjacent lens units, and in which the ratio between the total amount of movement of the first lens unit at the time of magnification change from a wide-angle limit to a telephoto limit and the focal length of the entire system at a telephoto limit is set forth.
Japanese Laid-Open Patent Publication No. 2008-209773 discloses a zoom lens having a four-unit construction of positive, negative, positive and positive and having an illuminance diaphragm located between the second lens unit and the third lens unit, in which in magnification change from a wide-angle limit to a telephoto limit, the first lens unit moves such as to be located on more object side at a telephoto limit than at a wide-angle limit, the second lens unit moves such as to be located on more image side at a telephoto limit than at a wide-angle limit, the illuminance diaphragm moves such as to be located on more object side at a telephoto limit than at a wide-angle limit, and the third lens unit moves such as to be located on more object side at a telephoto limit than at a wide-angle limit, and in which the ratio between the focal lengths of the entire zoom lens system at a wide-angle limit and at a telephoto limit is set forth.
Japanese Laid-Open Patent Publication No. 2008-225328 discloses a zoom lens having a four-unit construction of positive, negative, positive and positive and having an illuminance diaphragm located between the second lens unit and the third lens unit, in which in magnification change from a wide-angle limit to a telephoto limit, the first lens unit, the second lens unit, and the third lens unit move such that the interval between the first lens unit and the second lens unit increases while the interval between the second lens unit and the third lens unit decreases, and the illuminance diaphragm moves such that the distance from the second lens unit decreases, in which the second lens unit, in order from the object side, composed of three lenses consisting of a first negative lens, a positive lens, and a second negative lens, and in which the ratio between the focal length of the second lens unit and the focal length of the entire zoom lens system at a telephoto limit and the ratio between the focal length of the second negative lens and the focal length of the entire zoom lens system at a telephoto limit are set forth.
In the zoom lenses disclosed in the patent documents, size reduction is achieved to an extent applicable to thin and compact digital cameras. Nevertheless, in some cases, although a somewhat wide view angle is obtained at a wide-angle limit, the achieved zooming ratio is as low as 4 or the like. In alternative cases, in contrast, although a satisfactory zooming ratio of 6 or the like is obtained, the achieved view angle at a wide-angle limit is unsatisfactory. As such, from the perspective of the balance between the view angle at a wide-angle limit and the zooming ratio, these zoom lenses do not satisfy the requirement for digital cameras in recent years.